In recent years the search for oil and gas fields has spread to waters ever deeper offshore of the mainland. As the depth of water becomes greater, drilling has been done more frequently with the aid of floating drills instead of from fixed platforms, for reasons of cost. Particularly, drills that can be dynamically placed have been used, thus avoiding trouble with anchoring systems.
Drilling in deep water brings up a host of difficulties Which has served to highlight the inefficiency of the equipment usually employed in such deep water operations. However, the system of bases that make up the well-head now used in deep water drilling is quite different from that used for shallow water drilling, having been adapted so as to make operation with dynamically placed drills easier since they do not need guiding cables.
Another point which makes drilling difficult in deep water is the question of the degree of tilt of the base, which tilt will bring about a like tilt in the high pressure housing. Since a high pressure housing is used as the support for the blow-out control equipment, the tilt may make the blow-out control operate under stress and thereby lead to premature and undesirable wear of its parts. Therefore control over the tilt of the base system must be exercised. In deep water, such control can be accomplished by means of television cameras, since divers cannot be employed, and in order to carry out such monitoring, visibility must be good so that corrections and other adjustments may be made at the surface by remote control. Such cameras can be installed in the column around which the bases are fixed, as in remote control vehicles. This question of visibility has been taken into account since in the previous base systems gravel would gather at points where visibility had to be best, since the bases are low and return flow would take place through the mouth of the funnel. Therefore, guide-funnels were increased considerably in size and return was made sideways to prevent level gauges from becoming covered over with gravel. Thus, by altering the size of the funnel-guides as well as by altering the base system, it became possible to better place other equipment during installation.
However, improvements in the previous state of the art had not overcome problems in connection with working the wells regarding the next stage of preparation of wells for such purpose.
Among the principal difficulties of previous methods was that of the guide-funnel mentioned above. Because of its greater size (although this made it easier to install the blow-out control equipment), it was difficult to put in wet Christmas trees (which is an art-recognized expression for production well valve control systems). The guide-funnel therefore had to be withdrawn in order to install a wet Christmas tree. On the other hand, without a guide-funnel other difficulties in the deep-water placing of equipment arise, particularly regarding coupling tolerances, especially of the parts that make up the wet Christmas tree.